Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are typically inserted through an insertion opening of an ink loader for the printer, and the ink sticks are pushed or slid along the feed channel by a feed mechanism and/or gravity toward a heater plate in the heater assembly. The heater plate melts the solid ink impinging on the plate into a liquid that is delivered to a melt reservoir. The melt reservoir is configured to maintain a quantity of melted ink in liquid or melted form and to communicate the melted ink to one or more printheads as needed.
In order to prevent the ink storage and supply assembly 400 of the imaging device from exhausting the available supply of ink, the reservoirs 404 of the ink storage and supply assembly 400 may be provided with ink level sensors. Recently, ink level sensors have been developed that enable a continuous measurement of the level of ink in the reservoirs of the printer. These ink level sensors include a lower probe positioned near a lower portion of the reservoir, an upper probe that extends upward form the lower probe toward the top of the reservoir, and an outer probe. To detect the level of ink in an ink reservoir, an AC signal is driven to the outer probe. The ink in the reservoir conducts the AC signal to the lower probe and to the upper probe. A current flow is detected from the outer probe through the ink to the lower probe and from the outer probe through the ink to the upper probe. Assuming that the ink temperature and conductivity remains relatively consistent, a substantially constant current flow is detected via the lower probe. Varying levels of current flow are detected via the upper probe as more or less of the upper probe's surface area is covered or uncovered in ink. A continuous measurement of the height of ink in the ink reservoir may then be determined by comparing the varying current flow in the upper probe to the constant current flow in the lower probe.
The ink level sensor described above is robust to variation in ink conductivity that may result due to normal variation in the manufacturing processes of the ink and/or due to natural variation in the ink components. For example, due to variation inherent in the manufacture of ink from raw components, a moderate variation in the conductivity of the ink may be expected from batch to batch and accounted for accordingly. However, if ink having a conductivity that exceeds the range of reliable operation of a level sensor enters the reservoir, the level readings generated by the level sensor for that reservoir may not be accurate or the level sensor may fail altogether resulting in various printhead failures, including introduction of air which causes jetting failure, and weeping of jets which can contaminate the drum.